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<br />. <br />-- <br />'. .' <br />-. <br />;: i <br /> I 1 <br /> I i <br />, <br />.! <br />,. <br /> <br />O " ,'. G <br />~ l. ) <br /> <br />1/ <br /> <br />" <br /> <br />9 <br /> <br />and the u.s. Bureau of Reclamation made in 1948. It is Uo"lder:!!'tood <br /> <br />that the u.s. Geological Survey is now conducting a regional transient- <br /> <br />state study. When the results of this analysis are available in t..."le <br /> <br />fall, they will throw considerable light on ~~e uplift problem at ~~~ <br /> <br />Narrows Dam site.. <br /> <br />MATERIALS FOR CONSTPlJCTION <br /> <br />JU1 abWldance of fine material tor the core of the embankment is <br /> <br />available close to the dam site. The preferred location seems to be <br /> <br />on the north bank. i\ backhoe pit dug in this material sho.....ed less than <br /> <br />6 feet of core material resting on a water-bearing sandy strat~. The <br /> <br />IOOvement of water in the sand was noticeable and caused the so:.l above <br /> <br />to slough off. The stripping of this thin layer 0':1 the north bank <br /> <br />would cover a broad area and necessitate installing an efficient <br /> <br />drainage system during construction. <br /> <br />The south bank also contains suitable core material that i~ <br /> <br />more uniform. than that on the north sic:!e. However, this relatively <br /> <br />impervious alluvial blanket forms a partial seal over the highly <br /> <br />per.meable sands and gravels below. Stripping ~~ese fine sedirne~ts near <br /> <br />the dam would result in a shorter route for the reservoir water to ~~e <br /> <br />aquifers below and cause a steeper hydraulic gradient to he established <br /> <br />with accompanying higher uplift pressureS. <br /> <br />Pervious materials cu:e present in the South Platte riverbed <br /> <br />upstream from the proposed dam. 'They are essentially well-graded sanes <br /> <br />with some gravel. It would be desirable to have a larger percentage of <br />